These embodiments relate to devices employing rotors, such as helicopters and wind turbines.
Normally in an aircraft employing an airfoil to provide lift, it is desirable to prevent a stall from occurring. A stall generally occurs when the angle of attack of the airfoil exceeds the critical angle of attack at a particular speed. The critical angle attack is the point on the lift coefficient vs. angle of attack curve of the airfoil where maximum lift is achieved. After this point, increasing the angle of attack reduces the lift sharply, generally as a result of the air boundary layer separating from the surface of the airfoil. Stalls are often talked about in terms of airspeed, or the speed of the air flowing across the airfoil, however, because as airspeed decreases, the angle of attack required to produce a given amount of lift (i.e., the aircraft's weight) increases. The above description applies to both the wings of a fixed-wing aircraft such as a plane and to the blades of a rotary wing aircraft such as a helicopter, because both the plane's wings and the helicopters rotors use airfoils to generate lift. In the case of a typical helicopter employing a single main rotor, however, the left and right sides of the rotor during forward flight have different effective airspeeds. In a helicopter in which the rotor rotates counterclockwise when viewed from above, for example, the left tip of the rotor (the retreating blade) sees its tip airspeed minus the craft's forward speed, while the right tip of the rotor (the advancing blade) sees its tip airspeed plus the craft's forward speed. Therefore, when the craft's forward speed (or even the speed of a gust of wind, for that matter) is a significant fraction of the tip speed, a stall can occur on the retreating blade, resulting in a violent shift in the craft's attitude (generally a nose up due to gyroscopic precession and then a roll toward to the retreating side). Generally, only exceptional pilots at high altitudes can hope to recover from such an event. Therefore, helicopters have a V-ne (velocity never exceed) speed limit pilots are warned to obey at all times that is sufficiently low relative to the tip speed to prevent a retreating blade stall from occurring. To prevent the V-ne from being excessively low (rendering the helicopter too slow to perform usefully or incapable of flying in typical winds), the rotor tip speed is kept high, typically on the order of 450 mph. That way, a 100 mph forward speed or a 100 mph wind only changes the effective tip speed by less than 25%.
Having such a high tip speed creates other problems, however. The advancing blade of the rotor may approach supersonic speeds during high speed flight, causing shockwaves that disrupt airflow over the blades, destroying lift. Additionally, the high tip speed causes large centrifugal loads on the blades and rotor hubs that require additional material weight and aerodynamic drag to provide the requisite strength.
Pilots of conventional helicopters are in fact trained to maintain the RPM (revolutions per minute) of the rotor in a narrow range, preventing more than a few percent of variation through adjusting the pitch of the rotor blades or the power delivered from the helicopter's engines.
The high tip speed of a conventional design generally to avoid a stall also results in significantly lower power efficiencies in terms of the power required to lift a given amount of weight. For example, the lift a rotor produces is proportional to the square of its speed, but the power it consumes to do so is proportional to the cube of its speed. For example, increasing the speed of a rotor by a factor of 10 would increase lift by a factor of 100, but it would also simultaneously increase the power required by a factor of 1000!
The high tip speed also results in excessive noise, a great concern both to military aircraft for stealth reasons and to civilian aircraft flying close to populated areas.
The high tip speed can also result in injury or death to a person hit by the rotor.
The high tip speeds also may create a turbulent wake behind a whirling blade, which may reduce the efficiency of another blade of the same rotor following in the same path.
Finally, the high tip speed results in turbulent downdraft and vortices from the rotor that can endanger the helicopter during certain conditions, such as when the helicopter descends through its own downwash at a high rate.
Therefore, we can say that the danger of a stall is perceived as such a serious threat that many additional complications arise and performance sacrifices must be made through efforts to avoid it. It would be highly desirable to overcome these necessary evils.
It would also be advantageous to avoid the need for a tail rotor, as conventional helicopters using a single rotor driven by an engine in the airframe need to counterbalance the reaction force of turning the rotor. Estimates on the additional power consumed for the tail rotor to balance the torque from the main rotor range from 5-30%. Having to constantly balance the main rotor torque results in the need for almost constant pilot input to adjust for small wind shifts and changes in aerodynamic effects as the helicopter maneuvers, however, which we perceive as a much more serious consequence of having a tail rotor, making the aircraft much more difficult to control from the pilot's standpoint.
A few helicopters have used coaxial twin rotors to try to mitigate some of the aforementioned deficiencies. The coaxial configuration has two rotors along the same axis, rotating in opposite directions. However, up until this point, helicopters using such a configuration are otherwise exceedingly conventional, using 2-3 blades per rotor, high tip speeds, blade angles of attack tied together in some way, and so on. As a result, the coaxial configurations built to date have not shown advantages sufficient in the market to displace in any quantity the standard main rotor/tail rotor configuration used on nearly all (>95%) of helicopters.